Under Smart Cut™ technology, the splitting of a substrate can be influenced by numerous factors such as the type of implanted ions, implantation energy, implantation dose, implantation current (or current density i.e. the ratio of current to implanted surface area) and implantation temperature.
The implanting conditions of H+ ions compatible with splitting are known and proven for silicon, in particular for heat-activated splitting. Implantation energy is chosen in relation to the thickness of the layers it is desired to transfer. The implantation dose depends on implantation energy: the minimum dose typically varies between 4E16 H/cm2 and 6E16 H/cm2 when the energy varies between 30 keV and 210 keV. The implantation current can be modified between a few μA and a few mA without major impact on the splitting step: the sole influences of implantation current on the process are a slight variation in the splitting heat treatment to be applied and/or in the transferred thickness and/or in the roughness of the surface obtained after splitting. Similarly, the implantation temperature i.e. the temperature reached in the substrate at the weakened plane formed by the implanted species, may vary between −190° C. and +300° C. without having any blocking impact on the splitting step; as is the case for implantation current, the implantation temperature of the process only has a slight influence on the splitting heat treatment to be applied and/or on transferred thickness and/or on the roughness of the surface obtained after splitting.
In the case of InP, it is known from different documents that the splitting and quality of the transferred films is highly dependent on implantation temperature: for example in the article “Low temperature InP layer transfer” by Q.-Y. Tong, Y.-L. Chao, L.-J. Haung, and U. Gösele, Electron. Lett. 35, 341 (1999), it is indicated that the optimal implantation temperature for InP is in the order of 150 to 200° C., whilst in “Temperature dependence of hydrogen-induced exfoliation of InP” by S. Hayashi, D. Bruno, M. S. Goorsky, published in Appl. Phys. Lett., Vol. 85, No. 2, 12 Jul. 2004, p. 236-238, it is indicated that the optimal temperature is −20° C. These differences in evaluation may derive from parameters not controlled by the authors of these articles such as doping of the substrates, the power supplied by the implanting beam, thermal contact of the substrates during implantation. In addition, since implantation is conducted under vacuum, it is very difficult to measure and maintain the temperature of the implanted substrates throughout the implanting step which may last several hours.
Yet experiments by the Applicant have shown that the optimal implantation temperature of hydrogen to promote heat-activated splitting in InP is between 120 et 180° C. However, when available InP substrates are implanted using a conventional microelectronic implanter, the implantation temperature exceeds 200° C. (e.g. 205° C. for implantation at 20 keV and 100 microamperes on a substrate 50 mm in diameter) and on this account subsequent heat-activated splitting is no longer possible.